Agents which accelerate fetal lung maturation are being used clinically for the prevention of the respiratory distress syndrome of the newborn. An organ culture model of 19 day fetal rat lung has been developed. This model will be used to investigate the influence of aminophylline, thyroxin, corticosteroids and insulin on the biosynthesis of the pulmonary phospholipids in order to evaluate the influence of these hormones on fetal lung maturation. The ontogeny of the response on these hormones will also be determined as well as their effects on glycogen metabolism. Phospholipid and glycogen biosynthesis will be studied by measuring the incorporation of labeled precursors into phospholipid and glycogen in the explants. The activity of enzymes related to lung phospholipid and glycogen metabolism will also be studied as well as the phospholipid composition and content of the explants. Parallel light and electron microscopic studies will provide information on the morphologic effects of these agents on the developing lung.